The present invention relates generally to turbine engines, and more particularly, to systems and methods for use in securing buckets to a turbine engine rotor wheel assembly.
At least some known turbine engines, such as gas turbines and steam turbines, use axial entry buckets, i.e., rotor blades that are coupled to a rotor wheel by sliding the buckets generally parallel to the rotor axis into mating dovetail slots defined on the rotor wheel. Some known buckets include radial-inwardly projecting dovetails that mate in dovetail slots formed on the rotor wheel. The rotor wheel dovetail slots are circumferentially-spaced apart from each other about the periphery of the rotor wheel.
Some known turbine engines may also extend integral covers between circumferentially-adjacent buckets to dampen vibratory responses of the buckets and to increase the buckets' natural frequencies. The buckets each have a natural frequency at which it will resonate when excited. As buckets resonate, stresses in the buckets may rise and fall. Over time these oscillating stresses may cause the buckets to fail due to material fatigue. The magnitudes of the oscillating stresses in the buckets may be reduced and the bucket lives may be increased by increasing the natural frequencies and/or by damping the vibratory response of these parts. It may be desirable, however, that the buckets be tightly coupled at the bucket platforms in the circumferential direction to increase bucket natural frequencies, to reduce dynamic stresses in the dovetail, and to enable accurate standing assembled vibration test data to be gathered for tuning and frequency validation purposes.
In at least some known turbine engines that use integrally covered buckets, the buckets may be secured in the dovetail slots using keys located in grooves in the outer circumference of the rotor wheel and recesses in the sides of the buckets. A closure bucket may be secured to the rotor wheel using a dovetail segment that includes dovetails that extend generally opposite to each other. The rotor wheel may include a conventional dovetail slot that receives the dovetail segment. However, rather than a dovetail, the closure bucket may have a dovetail slot that accepts a dovetail of the dovetail segment. However, as the buckets are coupled about the rotor wheel using the dovetail system, the integral covers of the first and the next to the last assembled buckets may prevent insertion of the closure bucket. As a result, in at least some known turbine engines, keys cannot be used due to the need to move at least some of the buckets axially during insertion of the closure bucket.
In such known turbine engines, twist locks may be used to keep the buckets from shifting axially on the rotor wheel after assembly. The twist locks may be inserted in channels formed in the bottom of the dovetails. Prior to insertion of the closure bucket, the twist locks may be unlocked, to enable buckets adjacent to the closure bucket to be selectively moved apart. After the closure bucket is inserted into the rotor wheel, the twist locks may be relocked to prevent the buckets from moving axially on the rotor wheel. However, using twist locks increases the cost associated with such turbine engines and may also increase operating stresses induced to the rotor wheel assembly. Moreover, such twist locks do not enable tight coupling at the bucket platforms in the circumferential direction to raise bucket natural frequencies, and/or to reduced dynamic stresses in the dovetail.